feat(writers): stamp RelSpec version in generated headers; preserve DBML column order

- Add pkg/buildinfo with Version/BuildDate (set via ldflags, VCS fallback);
  cmd/relspec now sources version info from it
- Emit "RelSpec <version> (built: <date>)" in generated file headers for
  bun, gorm, drizzle, pgsql (incl. migration), mssql, sqlite writers
- pgsql writer: getSortedColumns now sorts by Sequence then Name so the
  streaming WriteSchema/WriteDatabase path preserves source column order
- dbml reader: mergeTable re-bases merged-in column Sequence values past the
  existing max, fixing colliding sequences (and alphabetical fallback) when a
  table is split across multiple DBML files
- Tests for bun/gorm header + column order, and dbml multi-file merge ordering

pkg/buildinfo/buildinfo.go

@@ -0,0 +1,49 @@
+// Package buildinfo exposes the RelSpec version and build date so that both the
+// CLI and the schema writers can stamp generated output with the same values.
+package buildinfo
+
+import (
+	"fmt"
+	"runtime/debug"
+	"time"
+)
+
+// Version and BuildDate are set via -ldflags at build time (see Makefile). When
+// built without ldflags they are backfilled from the Go module build info.
+var (
+	Version   = "dev"
+	BuildDate = "unknown"
+)
+
+func init() {
+	if Version != "dev" {
+		return
+	}
+	info, ok := debug.ReadBuildInfo()
+	if !ok {
+		return
+	}
+	var rev, vcsTime string
+	for _, s := range info.Settings {
+		switch s.Key {
+		case "vcs.revision":
+			if len(s.Value) >= 7 {
+				rev = s.Value[:7]
+			}
+		case "vcs.time":
+			vcsTime = s.Value
+		}
+	}
+	if rev != "" {
+		Version = rev
+	}
+	if t, err := time.Parse(time.RFC3339, vcsTime); err == nil {
+		BuildDate = t.UTC().Format("2006-01-02 15:04:05 UTC")
+	}
+}
+
+// GeneratedComment returns the one-line provenance string embedded in generated
+// files, e.g. "RelSpec dev (built: unknown)".
+func GeneratedComment() string {
+	return fmt.Sprintf("RelSpec %s (built: %s)", Version, BuildDate)
+}